Hitherto known systems for calibration of pressure transducers used in pressurization monitors for cleanroom use are usually pressurized relative to adjacent space to points of leakage. Single cleanrooms are often pressurized at 0.05″ water column according to surroundings. Pressurizing can be as low as 0.02″ water column and the highest pressure of 0.1″ water column is rare. Since present day deadweight pressure testers can only measure pressure in inches of water column and not the extreme low pressures of clean rooms they can't be used for calibration of clean room monitors. Instead the cleanroom monitors are usually subject to calibration by electronic devices that use secondary calibration and not the primary calibration traceable to NIST standards used by this invention that can measure pressures as low as 0.001″ water column.
The Extreme Low Air Pressure Deadweight Tester invention can be calibrated to be traceable to NIST standards. Deadweight testers are generally regarded as the most accurate method in calibration of pressure instruments, and often are used as a primary pressure standard. The operating principle is that a vertically free floating piston in a cylinder is pushed upward when air pressure acts on the weighted piston. The piston must float freely in its cylinder. The pressure in the circuit will be determined by the weights loaded on the piston divided by the affected area of the piston with corrections for temperature and local gravity. The problem is that today's testers will not work for extreme low pressures because air escapes between the piston and the cylinder and if an attempt is made to reduce air loss it results in high friction. Also piston has weight greater than the required low weight for extreme low pressure.
The Extreme Low Air Pressure Deadweight Tester has it's diaphragm type piston weight balanced out to zero weight which allows very low weights to be loaded on a diaphragm type piston for extreme low air pressure measurements. The use of a 0.002″ thick silicone rubber seal between a diaphragm type cylinder and a diaphragm type piston with a limited movement of a few thousandths of an inch result in no air loss and reduces the friction between piston and cylinder to almost zero. A pair of gold contacts that are normally closed will open when air pressure above the set value is applied which will turn off a red LED when the desired operation pressure is obtained. This Extreme Low Air Pressure Deadweight Tester has a range of 0.001″ water column to 1.00″ water column and can have an accuracy of 0.001% of the full range. The Extreme Low Air Pressure Deadweight Tester can be made for higher pressures as required.
The air pressure is determined by the weight 53 of FIG. 4, being placed on threaded rod 5 of FIG. 1 to load the piston 17 of FIG. 2 divided by the effective area of the piston. A piston with an effective area of 91.189 square centimeters when loaded with 231.4 grams will give a weight of 2.53756 grams which converts to 1.0″ water column (60 Deg. F.). To obtain weights for other inches of water it is only necessary to multiply 231.4 grams by inch of water column. Thus, for 0.5″ water column multiply 231.4 by 0.5 which gives the required weight loaded on the piston of 115.7 grams. Likewise for 0.001″ water column the required weight would be 0.001 multiplied by 231.4 or 0.2314 grams, etc. The weights can be traceable to NIST standards making the Extreme Low Air Pressure Deadweight Tester a primary standard.
Manometers using water or mercury can not detect extreme low pressures, because the density will vary with temperature, the fluid will adhere to the glass tubing, and measurement of the change in height of the fluid is difficult to detect at pressures of 0.001″ water column. Most electronic manometers are secondary standards using metal diaphragms with extremely small movement that must employ electronic amplifying to detect movements of 0.001″ water column.